This invention relates to a method and apparatus for isolating a portion of a wellhead during a fracturing operation.
One frequent well servicing technique for oil and gas formations having low permeability is to artificially “stimulate” to increase the permeability of the production zone(s). Generally, these stimulation techniques are referred to as “fracturing”. Fracturing involves pumping pressurized fluids through perforations in a well casing into a production zone in order to break or fracture pores in the production zone to improve permeability so that the hydrocarbon fluids can drain from the production zone into the casing. Fracturing generally involves first using a tool known as a perforating gun to perforate the production zone adjacent the casing. Thereafter, fracturing fluids are pumped under very high pressures of about 5,000-10,000 psi through the perforations into the formation. The high pressure breaks the formation to form a flow channel for hydrocarbon fluids. Proppants are also injected to prevent the formation from collapsing after the high stimulation pressure is released.
During fracturing, isolation tools are needed to isolate the wellhead from the high pressures of fracturing, since fracturing pressures are typically much higher than the wellhead pressure rating (which might be rated only at 5,000 psi, for example). In the prior art, these fracturing isolation tools generally seal inside the casing or on the bit guide in a manner which can restrict full bore access to the casing. Full bore access is particularly desirable for fracturing techniques which involve fracturing in stages. After fracturing, the fracturing isolation tool is removed. At this point, since the well may be live, it is necessary to maintain control over the well. One prior art approach is to install a bridge plug, which seals inside the casing. These tools are expensive to rent and to use. Another approach is to control the well pressure with a column of mud or water. However, this procedure can damage the formation. Both of the above approaches require a service crew at the well, which is time and resource intensive.
Fracturing isolation sleeves are shown in a number of patents, see for example U.S. Pat. Nos. 5,819,851; 6,247,537; 6,364,024; and 6,491,098 to Dallas, Canadian Patent 2,276,973 to Dallas, U.S. Pat. No. 4,993,488 to McLeod, U.S. Pat. No. 6,516,861 to Allen, and U.S. Pat. No. 6,920,925 to Duhn et al.
U.S. Pat. No. 7,069,987, filed Feb. 6, 2004, issued Jul. 4, 2006 to Kwasniewski et al., (assigned to the assignee of the present application), discloses a casing adapter tool to accommodate fracturing equipment at the wellhead during fracturing, and then to accommodate one or more pressure barrier seals in the wellhead, such as a check valve, after the fracturing operation. Full bore access to the production casing is preferably provided by this tool.
U.S. Pat. No. 7,308,934, filed Feb. 18, 2005, issued Dec. 18, 2007 to Swagerty et al., discloses a fracturing isolation sleeve for use in two wellhead members above a production casing. The fracturing isolation sleeve seals in the wellhead members above the production casing. As well, the sleeve bridges the two wellhead body members, i.e., is disposed in the internal bores of both of the wellhead body members. The wellhead members are typically a tubing head and an adapter. Further, the fracturing isolation sleeve has an internal diameter greater than or equal to the internal diameter of the production casing. The fracturing isolation sleeve is formed with a pressure barrier profile to seal a pressure barrier in its central bore.
The isolation sleeve of the Swagerty patent is directed at solving previous prior art problems which arise when the wellhead isolation tool seals to the inside surface of the casing string. In that previous prior art, the inside diameter of the wellhead isolation tool is substantially smaller than the inside diameter of the casing string. The bridge plugs, which are designed to have an outside diameter the same as the drift of the casing string, cannot pass through the wellhead isolation tool. Therefore, each time a bridge plug is installed, the wellhead isolation tool is removed and the wireline lubricator installed. Repetitive installation and removal of equipment adds to the costs of managing the wellhead.
However, a problem exists with the fracturing isolation sleeve of the Swagerty patent. During the fracturing operation within the fracturing isolation sleeve, the seal surfaces and the pressure barrier profiles formed for the later to be installed pressure barriers are both exposed to the fracturing environment, i.e., the high pressure and abrasion of the fracturing fluids. This exposure may damage the sealing surfaces and/or pressure barrier profile, preventing the pressure barrier from sealing after the fracturing process.